Exact travelling wave solutions of non-linear reaction-convection-diffusion equations -- an Abel equation based approach

We consider quasi-stationary (travelling wave type) solutions to a general nonlinear reaction-convection-diffusion equation with arbitrary, autonomous coefficients. The second order nonlinear equation describing one dimensional travelling waves can be reduced to a first kind first order Abel type differential equation By using two integrability conditions for the Abel equation (the Chiellini lemma and the Lemke transformation), several classes of exact travelling wave solutions of the general reaction--convection-diffusion equation are obtained, corresponding to different functional relations imposed between the diffusion, convection and reaction functions. In particular, we obtain travelling wave solutions for two non-linear second order partial differential equations, representing generalizations of the standard diffusion equation and of the classical Fisher--Kolmogorov equation, to which they reduce for some limiting values of the model parameters. The models correspond to some specific, power law type choices of the reaction and convection functions, respectively. The travelling wave solutions of these two classes of differential equations are investigated in detail by using both numerical and semi-analytical methods.Comment: 35 pages, 3 figures, accepted for publication in JMP. arXiv admin note: text overlap with arXiv:1409.060

Abel Dynamics of Titanium Dioxide Memristor Based on Nonlinear Ionic Drift Model

We give analytical solutions to the titanium dioxide memristor with arbitary order of window functions, which assumes a nonlinear ionic drift model. As the achieved solution, the characteristic curve of state is demonstrated to be a useful tool for determining the operation point, waveform and saturation level. By using this characterizing tool, it is revealed that the same input signal can output completely different u-i orbital dynamics under different initial conditions, which is the uniqueness of memristors. The approach can be regarded as an analogy to using the characteristic curve for the BJT or MOS transisitors. Based on this model, we further propose a class of analytically solvable class of memristive systems that conform to Abel Differential Equations. The equations of state (EOS) of the titanium dioxide memristor based on both linear and nonlinear ionic drift models are typical integrable examples, which can be categorized into this Abel memristor class. This large family of Abel memristive systems offers a frame for obtaining and analyzing the solutions in the closed form, which facilitate their characterization at a more deterministic level.Comment: 5 pages, 3 figure

A class of exact solutions of the Li\'enard type ordinary non-linear differential equation

A class of exact solutions is obtained for the Li\'{e}nard type ordinary non-linear differential equation. As a first step in our study the second order Li\'{e}nard type equation is transformed into a second kind Abel type first order differential equation. With the use of an exact integrability condition for the Abel equation (the Chiellini lemma), the exact general solution of the Abel equation can be obtained, thus leading to a class of exact solutions of the Li\'{e}nard equation, expressed in a parametric form. We also extend the Chiellini integrability condition to the case of the general Abel equation. As an application of the integrability condition the exact solutions of some particular Li\'{e}nard type equations, including a generalized van der Pol type equation, are explicitly obtained.Comment: 18 pages, no figures; minor revisions, accepted for publication in Journal of Engineering Mathematic

Uniform upper bounds for the cyclicity of the zero solution of the Abel differential equation

Given two polynomials $P,q$ we consider the following question: "how large can the index of the first non-zero moment $\tilde{m}_k=\int_a^b P^k q$ be, assuming the sequence is not identically zero?". The answer $K$ to this question is known as the moment Bautin index, and we provide the first general upper bound: $K\leqslant 2+\mathrm{deg} q+3(\mathrm{deg} P-1)^2$. The proof is based on qualitative analysis of linear ODEs, applied to Cauchy-type integrals of certain algebraic functions. The moment Bautin index plays an important role in the study of bifurcations of periodic solution in the polynomial Abel equation $y'=py^2+\varepsilon qy^3$ for $p,q$ polynomials and $\varepsilon \ll 1$. In particular, our result implies that for $p$ satisfying a well-known generic condition, the number of periodic solutions near the zero solution does not exceed $5+\mathrm{deg} q+3\mathrm{deg}^2 p$. This is the first such bound depending solely on the degrees of the Abel equation

On Moment Condition and Center Condition for Abel Equation

In this paper we consider Abel equation $x' = g(t)x^2+f(t)x^3$, where $f$ and $g$ are analytical functions. We proved that if the equation has a center at $x=0$, then the Moment Conditions, i. e., $m_k=\int_{-1}^1f(t)(G(t))^kdt=0,~~k=0,1,2$, is satisfied where $G(t)=\int_{-1}^tg(s)ds$. Besides, we give partial a positive answer to a conjecture proposed by Y. Lijun and T. Yun in 2001.Comment: arXiv admin note: text overlap with arXiv:1707.0266

Exact solutions of the Li\'enard and generalized Li\'enard type ordinary non-linear differential equations obtained by deforming the phase space coordinates of the linear harmonic oscillator

We investigate the connection between the linear harmonic oscillator equation and some classes of second order nonlinear ordinary differential equations of Li\'enard and generalized Li\'enard type, which physically describe important oscillator systems. By using a method inspired by quantum mechanics, and which consist on the deformation of the phase space coordinates of the harmonic oscillator, we generalize the equation of motion of the classical linear harmonic oscillator to several classes of strongly non-linear differential equations. The first integrals, and a number of exact solutions of the corresponding equations are explicitly obtained. The devised method can be further generalized to derive explicit general solutions of nonlinear second order differential equations unrelated to the harmonic oscillator. Applications of the obtained results for the study of the travelling wave solutions of the reaction-convection-diffusion equations, and of the large amplitude free vibrations of a uniform cantilever beam are also presented.Comment: 28 pages, no figures; minor modifications, accepted for publication in Journal of Engineering Mathematic

Exact analytical solutions of the Susceptible-Infected-Recovered (SIR) epidemic model and of the SIR model with equal death and birth rates

In this paper, the exact analytical solution of the Susceptible-Infected-Recovered (SIR) epidemic model is obtained in a parametric form. By using the exact solution we investigate some explicit models corresponding to fixed values of the parameters, and show that the numerical solution reproduces exactly the analytical solution. We also show that the generalization of the SIR model, including births and deaths, described by a nonlinear system of differential equations, can be reduced to an Abel type equation. The reduction of the complex SIR model with vital dynamics to an Abel type equation can greatly simplify the analysis of its properties. The general solution of the Abel equation is obtained by using a perturbative approach, in a power series form, and it is shown that the general solution of the SIR model with vital dynamics can be represented in an exact parametric form.Comment: 13 pages, 4 figures, accepted for publication in Applied Mathematics and Computatio

A self-similar inhomogeneous dust cosmology

A detailed study of an inhomogeneous dust cosmology contained in a $\gamma$-law family of perfect-fluid metrics recently presented by Mars and Senovilla is performed. The metric is shown to be the most general orthogonally transitive, Abelian, $G_2$ on $S_2$ solution admitting an additional homothety such that the self-similar group $H_3$ is of Bianchi type VI and the fluid flow is tangent to its orbits. The analogous cases with Bianchi types I, II, III, V, VIII and IX are shown to be impossible thus making this metric privileged from a mathematical viewpoint. The differential equations determining the metric are partially integrated and the line-element is given up to a first order differential equation of Abel type of first kind and two quadratures. The solutions are qualitatively analyzed by investigating the corresponding autonomous dynamical system. The spacetime is regular everywhere except for the big bang and the metric is complete both into the future and in all spatial directions. The energy-density is positive, bounded from above at any instant of time and with an spatial profile (in the direction of inhomogeneity) which is oscillating with a rapidly decreasing amplitude. The generic asymptotic behaviour at spatial infinity is a homogeneous plane wave. Well-known dynamical system results indicate that this metric is very likely to describe the asymptotic behaviour in time of a much more general class of inhomogeneous $G_2$ dust cosmologies.Comment: 17 pages, 3 postscript figures, uses psfig,sty, accepted for publication in Class. Quantum Gra

Limit cycles of planar system defined by the sum of two quasi-homogeneous vecter fields

In this paper we consider the limit cycles of the planar system $$\frac{d}{dt}(x,y)=\mathbf X_n+\mathbf X_m,$$ where $\mathbf X_n$ and $\mathbf X_m$ are quasi-homogeneous vector fields of degree $n$ and $m$ respectively. We prove that under a new hypothesis, the maximal number of limit cycles of the system is $1$. We also show that our result can be applied to some systems when the previous results are invalid. The proof is based on the investigations for the Abel equation and the generalized-polar equation associated with the system, respectively. Usually these two kinds of equations need to be dealt with separately, and for both equations, an efficient approach to estimate the number of periodic solutions is constructing suitable auxiliary functions. In the present paper we develop a formula on the divergence, which allows us to construct an auxiliary function of one equation with the auxiliary function of the other equation, and vice versa

On the integrability of the Abel and of the extended Li\'{e}nard equations

We present some exact integrability cases of the extended Li\'{e}nard equation $y^{\prime \prime }+f\left( y\right) \left(y^{\prime }\right)^{n}+k\left( y\right) \left(y^{\prime }\right)^{m}+g\left(y\right) y^{\prime }+h\left( y\right) =0$, with $n>0$ and $m>0$ arbitrary constants, while $f(y)$, $k(y)$, $g(y)$, and $h(y)$ are arbitrary functions. The solutions are obtained by transforming the equation Li\'{e}nard equation to an equivalent first kind first order Abel type equation given by $\frac{dv}{dy} =f\left( y\right) v^{3-n}+k\left( y\right) v^{3-m}+g\left( y\right) v^{2}+h\left( y\right) v^{3}$, with $v=1/y^{\prime }$. As a first step in our study we obtain three integrability cases of the extended quadratic-cubic Li\'{e}nard equation, corresponding to $n=2$ and $m=3$, by assuming that particular solutions of the associated Abel equation are known. Under this assumption the general solutions of the Abel and Li\'{e}nard equations with coefficients satisfying some differential conditions can be obtained in an exact closed form. With the use of the Chiellini integrability condition, we show that if a particular solution of the Abel equation is known, the general solution of the extended quadratic cubic Li\'{e}nard equation can be obtained by quadratures. The Chiellini integrability condition is extended to generalized Abel equations with $g(y)\equiv 0$ and $h(y)\equiv 0$, and arbitrary $n$ and $m$, thus allowing to obtain the general solution of the corresponding Li\'{e}nard equation. The application of the generalized Chiellini condition to the case of the reduced Riccati equation is also considered.Comment: 13 pages, no figures, accepted for publication in Acta Mathematicae Applicatae Sinica, English Serie